Caries detection methods

The detection of carious lesions has been primarily a visual process, based principally on clinical-tactile inspection and radiographic examination. Caries detection methods should be capable of detecting lesions at an early stage, when progression can be arrested or reserved, avoiding premature tooth treatment by restorations. However, none of the conventional methods fulfill this requirement and are highly subjective. The development of some alternative non-invasive detection methods, such as laser fluorescence devices (DIAGNOdent and DIAGNOdent pen), quantitative light-induced fluorescence (QLF), fluorescence camera (VistaProof), LED technology (Midwest Caries I.D.), fiber-optic transillumination (FOTI), digital imaging fiber-optic transillumination (DIFOTI) and electrical caries monitor (ECM), can offer objectives assessments, where traditional methods could be supplemented by quantitative measurements.

4.1 Visual-tactile examination

Visual changes of the dental structure resulting from the demineralization process can be visually observed during caries development, such as an increase in opacity and roughness of the enamel.

Visual examination has been widely used in dental clinics for detecting carious lesions on all surfaces. This method is based on the use of a dental mirror, a sharp probe and a 3-in-1 syringe and requires good lighting and a clean/ dry tooth surface (Hamilton, 2005). The examination is based primarily on subjective interpretation of surface characteristics, such as integrity, texture, translucency/opacity, location and color (Ekstrand et al., 1997; Nyvad et al., 1999). However, tactile examination of dental caries has been criticized because of the possibility of transferring cariogenic microorganisms from one site to another, leading to the fear of further spread of the disease in the same oral cavity. Moreover, use of an explorer can cause irreversible damages to the iatrogenic and demineralized tooth structure (Ekstrand et al., 1987; Stookey, 2005; Loesche et a!., 1979).

Tooth separation can be used as a method for examination of a suspicious area on the approximal surface. With this technique an orthodontic elastic separator can be applied for 2-3 days around the contact areas of approximal surfaces, facilitating the clinical and probing assessments. However, this method might create some discomfort and requires an extra visit (Araujo et al., 1996). Studies have shown that tooth separation have detected more non-cavitated enamel lesions than visual-tactile examination without separation or bitewing examination (Hintze et al., 1998; Pitts & Rimmer, 1992).

Nyvad's system (Nyvad et al., 1999) is a reliable method for activity assessment of non-cavitated and cavitated caries lesions. According to this system, the examination is based only on clinical features of the surface (color, opacity and presence of discontinuities or cavitations), classifying the lesion as inactive or active. The original system used biofilm accumulation as an indicator for caries activity and used a sharp dental explorer to assess surface roughness. However, the Nyvad system was modified; adopting the use of a ball-ended probe should be gently drawn across the surface in order to assess its texture (rough or smooth) and also to remove the biofilm (Braga et al., 2009). If the lesion is active and cavitated, operative treatment is recommended. If active and non-cavitated, non-operative, preventive treatment is recommended (Nyvad, 2004). For detecting carious lesions, the examination should be mainly based on careful visual assessment on a clean/ dry surface, without probing. An important aspect of caries detection is that the surface must be dry because saliva can mask differences in the reflection of light between carious and healthy tooth structure, hindering the observation of changes in color and brightness on the enamel surface. The criteria scores identify sound and active/inactive primary or secondary caries lesions. The Nyvad system has been shown to have good reproducibility and also construct and predictive validity for assessment of caries activity (Nyvad et al., 1999, 2003).

Visual examination has been show to have a high specificity but low sensitivity and reproducibility (Bader et al., 2001). Therefore, different criteria have been proposed to provide defined descriptors of different severity stages of caries lesions (Ekstrand et al., 1997; Ismail et al., 2007; Nyvad et al., 1999).

After the analysis of a systematic review presented in a conference in the USA and in the International Consensus Workshop on Caries Clinical Trials held in Scotland, it was concluded that the reliability and reproducibility of currently available caries detection / diagnostic systems, including visual and visual-tactile criteria, were not strong (Bader et al., 2001; Pitts & Stamm, 2002). Based on these findings, a new visual criterion has been introduced for caries detection.

The International Caries Detection & Assessment System (ICDAS) was developed and introduced by an international group of researchers (cariologists and epidemiologists) to provide clinicians, epidemiologists, and researchers with an evidence-based system for caries detection (Pitts, 2004). This method was devised based on the principle that the visual examination should be carried out on clean, plaque-free teeth, with carefully drying of the lesion / surface to identify early lesions. According to this system, the replacement of the traditional explorers and sharp probes with a ball-ended periodontal probe would avoid traumatic and iatrogenic defects on incipient lesions (Ekstrand et al., 2007; Ismail et al., 2007; Jablonski-Momeni et al., 2007).

ICDAS was developed with the mission to devise a set of international visual criteria for caries detection that would also allow assessment of caries activity (Ekstrand et al., 2007). The Lesion Activity Assessment (LAA) criteria have been developed for use in association with the ICDAS scoring system based on using weighted numerical values for lesion appearance (ICDAS score of the lesion), lesion location in relation to a cariogenic plaque stagnation area and surface integrity by tactile sensation when a ball-ended probe is gently drawn across the surface (Ekstrand et al., 2007; Varma et al., 2008). This evaluation involves the characterization of the caries lesion activity during a single clinical examination, in real time, in order to determine whether intervention is necessary (Ekstrand et al., 2009). The association of the LAA and the ICDAS codes involves lesion detection and coding, thereby estimating its depth or severity, and assessing its activity (Braga et al., 2009). An in vitro study found that there is no major difference between the Nyvad system and the ICDAS-LAA in assessing caries activity in primary teeth (Braga et al., 2009). However, in a clinical study ICDAS-LAA seems to overestimate the caries activity assessment of cavitated occlusal lesions in primary teeth compared to the Nyvad system (Braga et al., 2010).

Despite being the most widely used method in clinical practice, many studies have shown that visual-tactile examination should be associated with other caries detection methods, such as bitewing radiographs, especially for early caries lesions detection in approximal surfaces and for lesion depth evaluation on occlusal surfaces (Lussi, 1993; Lussi et al., 2006; Sanden et al., 2003; Wenzel, 2004).

4.2 X-ray based methods

The discovery of X-rays by Wilhelm Conrad Roentgen in 1895 provided a major advance in diagnostic imaging. In dental field, the North American dentist Edmund Kells began experimenting with radiography in 1986, becoming the pioneer of dental radiology.

Since then, the use of X-rays and radiographic films promoted a significant jump in the direction of dental therapy, since it provided substantial contribution in obtaining the diagnosis. In addition, radiographic techniques have been modified to acquire optimum X-ray quality and to increase diagnostic possibilities, as for detecting caries lesions.

4.2.1 Conventional radiography

Radiography is the most common caries lesion detection aid. It is fundamentally based on the fact that as the caries progress proceeds, the mineral content of enamel and dentin decreases, resulting in a decrease in the attenuation of the X-ray beam as it passes through the teeth. This feature is recorded on the image receptor as an increase in radiographic density. Clinically, the detection of carious lesions is based on a combination of visual-tactile and radiographic examination.

Bitewing radiography has been used for the detection and evaluation of caries lesions depth, which are invisible or poorly visible for inspection. Thus, radiography is mainly used for the detection of carious lesions in approximal surfaces, but is also recommended as a supplement for occlusal caries detection. However, experiments have shown that, once an occlusal carious lesion is clearly visible on radiographs, histological examination shows that demineralization has extended to or beyond the middle third of the dentin (Ricketts et al., 1995). Therefore, radiographic examination may underestimate the extent of caries lesions (Dove, 2001).

Bitewing radiography presents a tendency to make false-positive scores, and this could be due to the Mach-band effect, a perceptual phenomenon in which there is an enhancement of the contrast between a dark and a relatively lighter area, resulting in a dark band sharply demarcated (Berry, 1983). This effect causes an inclination to see radiolucency in the dentin-enamel junction where no dentin lesion is actually present (Espelid et al., 1994). Another effect, called cervical burnout, can be erroneously interpreted as cervical caries, once a collar or wedge-shaped radiolucency occurs between the bone height and the cemento-enamel junction (CEJ). This effect is an optical illusion phenomenon, due to the tissue density and the variable penetration of X-ray at the cervical region of the tooth and the regions above and below it, which produces a dark shadow on the radiograph due to lower absorption of photons in the neck of the tooth (Berry, 1983). For these reasons, radiographs should be interpreted with caution and requires constant retraining, updating, experience and information of the human observer (Diniz et al., 2010).

Several criteria are used to classify the extent of carious lesions on radiographs, such as (0) absence of radiolucency, (1) radiolucency in the outer half of the enamel, (2) radiolucency on the inner half of the enamel, which can extend up to the dentin-enamel junction (DEJ), (3) radiolucency in the outer half of the dentin and (4) radiolucency in the inner half of the dentin toward the pulp chamber (Mejare & Kidd, 2008).

Regarding the performance of bitewing radiography, studies have found that the X-rays show a high sensitivity (50-70%) to detect caries lesions in dentin of both approximal and occlusal surfaces, compared to clinical visual detection. However, the validity of detecting enamel lesions is limited on the approximal surfaces and low for the occlusal surfaces (Wenzel, 1995, 2004). This difference can be explained by the fact that radiography is a 2-dimensional image of a 3-dimensional anatomy of the tooth structure. So, the superimposed cuspal tissues obscure initial changes in occlusal surfaces (Espelid et al., 1994; Neuhaus et al., 2009). In a systematic review of the literature, the evidence suggests that radiographs have high specificity and low sensitivity for caries detection. In other words, this means that there are great chances to occur false-negative diagnosis in the presence of caries than false-positive diagnosis in the absence of disease (Dove, 2001).

It is important to stress that many different factors can affect the ability of bitewing radiography to accurately detect lesions, such as technique, image processing, type of image receptor, exposure parameters, vertical and horizontal angulations of the X-ray beam, positioning of the film, display system, viewing conditions, possible distortions caused by the structures attached to the dental tissues and failures of interpretation, which can lead to an incorrect diagnosis (Dove, 2001).

Radiographic examination is useful in monitoring caries lesion development, in view of the fact that non-cavitated lesions can be reversed by non-invasive intervention, providing changes in mineral content of dental tissues. However, there are limits to the radiographic examination which should be considered, particularly since lesion behavior has changed, with cavitation occurring much later than previously (Pitts & Rimmer, 1992), Thus, it is worth remembering that radiography is not able to differentiate between an active and an arrested caries lesion, and to distinguish a cavitated and a non-cavitated lesion. According to Ratledge et al. (2001), 50-90% of dentin caries lesions radiographically observed on the approximal surface might present cavitation. There are cases where clinically "sound" and apparently intact occlusal surfaces, however, may develop lesions which penetrate into the dentin, sometimes named "hidden" caries (Ricketts et al., 1997), which can be observed only through radiographic examination (Figure 4).

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Fig. 4. Detection of occlusal caries. (A) Clinical aspect of a lesion in an intact surface. (B) Radiographic aspect of the lesion penetrating into the dentin - a typical "hidden caries".

Currently, there are some questions regarding specific indication of radiographs and intervals between subsequent radiographic examinations for caries detection. There is no evidence that routine radiographs will benefit a low caries risk population. In fact, this procedure can be harmful because it can induce a great risk of overdiagnosis, and consequently, an overtreatment. The frequency of taking radiographs depends on the individual caries risk, lesion activity and on the individual benefit to a patient (Neuhaus et al., 2009).

4.2.2 Digital radiography

Digital radiography is a complementary method that has been available in dentistry for more than 25 years, but digital imaging has not replaced conventional film-based radiography completely. Studies have shown that the number of dental professionals using digital radiography in clinical practice range from 11% to 30%. This fact can be attributed to the financial investment required to replace conventional radiography with digital imaging and also for the hesitancy to use a new technology, since it requires additional training on basic computer skills. On the other hand, a professional who is starting his/her career will not find huge differences in costs to acquire a conventional or a digital radiography system. Practitioners should remember that conventional radiography also involves costs for items, such as radiographic films, film mounts, processing solutions and time needed for cleaning the film processor (van der Stelt, 2008).

Studies have shown many advantages of digital radiography compared with conventional radiography. These include image acquisition process in real time, since the image is displayed immediately after exposure and no processing had to be performed. Other benefits include reductions in radiation dose (between 5% to 50% of the dose needed for conventional radiography) to obtain quality diagnostic images, time savings and digital manipulation of the image to enhance viewing, avoiding unnecessary or repeated radiographs. Digital images facilitate communication and case discussion among dental professionals, being a visual aid to be shown to the patient on the computer screen, increasing the confidence and credibility in the treatment-decision making process. However, the primary disadvantages of digital systems include the rigidity and thickness of the sensors, the high initial system cost and unknown sensor lifespan (Bin-Shuwaish et al., 2008; van der Stelt, 2008; Wenzel, 1998).

It is imperative to understand the digital radiography system to understand the principle of image manipulation. A digital image consists of a set of cells that are ordered in rows and columns, forming a table. Each cell is characterized by three numbers: the x-coordinate, the y-coordinate and the gray value. The gray value is a number that corresponds with the X-ray intensity at that location during the exposure of the digital sensor. Individual cells are called "picture elements", which had been shortened to "pixels". The numbers describing each pixel are stored in an image file in the computer. This feature is an essential difference between conventional and digital radiographs, once digital images can be modified after they have been produced. Thus, the user can apply mathematical operations (special algorithms or filters) to modify the pixel values, improving the image quality and modifying other characteristics, such as zoom, contrast, density and brightness of an image. The image numbers are converted into gray values and these are displayed on the computer screen as analog data. Then, the professional can assess and interpret the radiographic image produced (van der Stelt, 2008; Wenzel, 1998).

An example of useful image manipulation is the optimization of contrast and brightness of an image. This technique can be used to correct overexposure or underexposure of an image, although it is not an excuse to not pay attention to the correct exposure parameters. The manipulation can help to recover an image in which the exposure conditions were not optimal. This procedure may prevent the need for a radiograph remake, protecting the patient from an extra dose of radiation (van der Stelt, 2008).

Digital image presents lower spatial resolution when compared to the image obtained by conventional radiography. The extension or palette for digital images is normally limited to 256 shades of gray, while more than a million shades of gray may appear for conventional X-ray film. Therefore, it can be speculated that the performance of digital radiography for caries detection would not be superior to that of conventional radiography. However, the performance of digital radiography for caries detection can be improved with image manipulation possibility, such as contrast modification. Thus, digital radiography systems seem to be as accurate as the conventional radiography system. According to a literature review, digital radiography showed high sensitivity for detecting occlusal caries lesions into dentin (60-80%), with false-positives results of 5-10% (Wenzel, 1998).

Undoubtedly, as technology evolves, it is supposed that the performance of digital radiography will be improved in a near future. The development of different sensors and software will support the reliability and viability of digital radiography applications by dental professionals, bringing this method to daily practice.